1. Technical Field
This disclosure relates to damping in superconducting levitation systems. Particularly, this disclosure relates to damping in high-temperature superconducting bearings in levitation systems, such as used to support a flywheel energy storage system.
2. Description of the Related Art
Superconducting levitation systems, and specifically those employing superconducting bearings, are typically implemented with a superconducting stator (non levitated component) and a permanent magnet rotor (or levitated component). The superconductor is most commonly a bulk, high-temperature superconductor. Superconducting bearings of this type are useful because they can be used to form a passively stable levitation system with extremely low rotational losses. Such bearings have particular application to high-efficiency flywheel energy-storage devices. Superconducting bearings and high-efficiency flywheels have been subjects of past developments.
FIG. 1A illustrates a conventional permanent magnet high-temperature superconducting levitation system 100. The superconductor element 102 (such as a stator) is coupled to a cold source 104 which maintains its temperature at a level to support superconductivity. A thermal insulator 106 isolates and supports the superconductor element 102 and the cold source 104 relative to a ground state 108. A magnetic field generated by the superconductor element 102 supports the permanent magnet 110 (e.g., a rotor spinning at rate ω) levitating it above the superconductor element 102 with force F. In this example system 100, the permanent magnet 110 may be coupled to a larger structure and may comprise a more complex magnetic structure. The high-temperature superconductor element 102 may comprise one or more bulk crystals of yttrium barium copper oxide (YBCO) or any other known high-temperature superconductor material. Due to flux pinning in the high-temperature superconductor, the orientation of the central axis may be in any direction. For example, the permanent magnet could rotate below the high-temperature superconductor.
FIG. 1B illustrates another example of a conventional permanent magnet high-temperature superconducting levitation system 120. In this example, the permanent magnet 122 is shown levitated in vacuum enclosed by the surrounding vacuum chamber 124. The high-temperature superconductor 126 is situated inside a cryochamber 128 on a support 134 and bathed in a pool liquid nitrogen 130. One or more low thermal conductance mechanical supports 132 separate the cryochamber 128 that houses the high-temperature superconductor 126 from the fixed support 136 of the environment. The supply of liquid nitrogen may be regulated in some way through an inlet 138 and outlet 140 (e.g., fed via a pressure regulator from a pressurized Dewar, part of a thermosiphon loop, etc.). The cryogenic chamber 128 should be thermally isolated from the environment. It resides in a substantial vacuum, and radiation losses may be reduced by wrapping the chamber 128 with one or more layers of multi-layered insulation (MLI). MLI is typically a very thin sheet of mylar film with an even thinner film of aluminum evaporated onto it as is known in the art. The use of MLI is a standard practice in cryogenic technology. In addition, the mechanical supports 132 that connect the cryogenic chamber to the fixed support 136 of the environment should be of low thermal conductance; the thermal conduction of heat through the supports 132 should be reduced while continuing to provide sufficient mechanical strength.
In prior art systems, the supporting structures are rigid; the coupling between the holder and the ground is modeled as perfectly rigid because the stiffness is so high. It is convenient to make this stiffness high, because a center of geometry is defined for the system in doing so. As a consequence, there is the equivalent of a relatively stiff spring between the actual surface that holds the superconductor element and the ground. Most other mechanical bearing systems can not tolerate the large radial excursion that a superconducting bearing can, so movement is restricted in conventional systems. The stiff spring results in a high resonant frequency for vibrations of the superconductor element holder relative to ground. There is also a small amount of structural damping inherent in this rigid structure. When applied to the levitation system as a whole, the damping to the superconductor element holder is small. In addition, because the resonant frequency between the superconductor element holder and ground is higher than the resonant frequency between of the superconductor element holder and the magnet, any coupling of this damping to the levitated magnet is small.
One general difficulty in developing superconducting bearings and supercondcuting levitation systems arises from the inherently low damping of the bearing itself, especially at small vibrational amplitudes. The primary damping in a superconducting levitational system is due to magnetic hysteresis in the superconductor. To a first order, the cyclic energy loss of the system is proportional to the cube of the displacement from equilibrium and inversely proportional to the critical current density of the superconductor. Large criticial current densities are desirable to increase the levitational force and to decrease the amount of superconductor required. This system is particularly ineffective at damping small amplitude oscillations and whirls of levitated rotors.
Techniques to increase damping, such as applying eddy current dampers, can be used with superconducting bearings. For example, eddy current dampers could be employed by placing a copper sheet in close proximity to a levitated permanent magnet. Unfortunately, in rotating systems such dampers cause high rotational loss, as the inherent azimuthal magnetic field of the rotating magnet induces eddy currents that develop forces to oppose the rotation. This and other existing methods to increase damping cause much higher rotational loss, which reduces or negates the primary benefit of the superconducting bearing. Another technique is to combine an active magnetic bearing with the superconducting bearing to increase the damping as needed. This technique also increases the losses in the bearing system.
In view of the foregoing, there is a need in the art for apparatuses and methods to increase the damping of a superconducting bearing without sacrificing the low rotational loss. In addition, there is a need for such apparatuses and methods to operate with flywheel energy storage systems. There is further a need for such systems and apparatuses in space applications. These and other needs are met by the present disclosure as detailed hereafter.